As proposed in U.S. Pat. No. 4,993,387 (JP 2605805), for instance, a conventional knock determination apparatus determines a distribution of a value of vibration intensity or magnitude calculated from an output signal of a knock sensor (vibration intensity distribution) and calculates a knock reference value KCK from the vibration intensity distribution. This knock reference value is calculated as shown in (a) of FIG. 6. Specifically, a central value VMED as a statistical index for evaluating the shape of the vibration intensity distribution and a standard deviation σ are calculated. The knock reference value KCK is determined by adding a value 3σ, which is three times the standard deviation σ, to the central value VMED.KCK=VMED+3σ
As shown in (b) of FIG. 6, a position of the vibration intensity distribution relative to the vibration intensity varies due to variations in a component such as an engine or a knock sensor. This difference in position is developed as offset. However, the shape of the vibration intensity distribution is maintained as substantially the same, regardless of the variations in the component. As a result, when the knock reference value (KCK=VMED+3σ) is calculated by using the central value VMED of the vibration intensity distribution as a reference as described above, the knock reference value KCK can be set in such a manner as to eliminate the influence of the variations in the component. This method is advantageous in performing a knock determination without influence of the variations in the component.
On the other hand, as shown in (c) of FIG. 6, in an operating condition where a knocking frequently or recurrently occurs, both the central value VMED of the vibration intensity distribution and the standard deviation σ increase resulting in divergence. Therefore, in the operating condition where the knocking frequently occurs, the knock reference value KCK becomes very larger as compared to that at the time of the absence of the knocking. As a result, although the knocking frequently occurs in fact, knocking may not be determined as occurring.